(a) Field of the Invention:
This invention relates to a photosetting resin composition. More specifically, the present invention is concerned with a photosetting resin composition suitable as a coating material for optical transmission glass fibers (will hereinafter abbreviated as "optical fibers"). The photosetting resin composition has a high setting speed and permits the formation of a coating film of a low modulus of elasticity. Moreover, the modulus of elasticity of the coating film has small temperature dependency at low temperatures.
(b) Description of Related Art:
Optical fibers are generally applied with a primary coat of a high molecular material immediately after their drawing, so that they will be protected physically and chemically from the external environment. As such primary coating materials, silicone resins and various ultraviolet setting resins have been used primarily to date.
From the standpoint of optical transmission loss, a primary coating material for optical fibers is required to have a low modulus of elasticity and low glass transition temperature, and the temperature dependency of its modulus of elasticity is supposed to be small at low temperatures. On the other hand, a higher setting speed is desired to improve the productivity of optical fibers from the economical standpoint. Silicone resins have a low modulus of elasticity and a low glass transition temperature and hence have superb transmission characteristics. They cannot however improve the productivity due to their slow setting speed, so that they are economically inferior. Silicone resins are accompanied by another drawback that the transmission loss becomes greater along the passage of time because of substantial evolution of hydrogen gas from them.
It has also been proposed to use ultraviolet curable coating materials such as epoxy acrylates, urethane acrylates and polybutadiene acrylates. These coating materials are however accompanied by a drawback that those having a smaller modulus of elasticity generally have a slower setting speed. They are accompanied by another drawback that the temperature dependency of their moduli of elasticity at low temperatures are great, so that at low temperatures, their moduli of elasticity become greater and the transmission loss increases accordingly. Reference may be had to U.S. Pat. No. 4,694,052 in this regard.
Photosetting resin compositions composed of 20-80 wt. % of a specific urethane acrylate and 80-20 wt. % of a particular monofunctional acrylate have also been known from U.K. patent No. 2,163,443A. Their setting speeds, the temperature dependency of their moduli of elasticity and transmission losses were however not taken into consideration in the above U.K. patent.